Various mechanisms have been developed for limiting nitrous oxide emissions from internal combustion engines.
As it is well known, nitrogen oxides (also known as NOx) are of particular concern in lean-burn, diesel engines, and devices such as NOx traps—also known as NOx adsorbers or Lean NOx Traps (LNT)—and Selective Catalytic Reduction (SCR) systems have been developed for this purpose.
A NOx trap assists in reducing NOx emissions by storing nitrogen dioxide as nitrates in zeolite adsorbers during fuel-lean conditions and releasing the nitrates as nitrogen oxides and oxygen during fuel-rich conditions, which are then converted into N2 and H2O. In diesel engines, a lean NOx trap conventionally combines the NOx adsorbing function with an oxidation catalyst function. Hence, the LNT generally consists of a single housing with adsorbing catalyst material and oxidation catalyst material (to provide the functions of a diesel oxidation catalytic converter). Alternative configurations include a NOx trap arranged downstream of a diesel oxidation catalytic converter (DOC).
NOx traps significantly reduce NOx emissions, but have a susceptibility to sulfur poisoning. Indeed, sulfur is present in fuel and engine oil and tends to bind to nitrate sites on the zeolite adsorber in the form of sulfates SO4. Because sulfates are more stable than nitrates and carbonates, the sulfur species are not released during the fuel-rich regeneration process that is performed to release the carbon dioxide and nitrogen oxides, i.e. in the regular operating range for NOx adsorbers at 150 to 500° C.
Various processes have been developed to desulfurize NOx traps. One conventional approach is to control the engine so that the NOx trap temperature reaches an appropriate temperature of generally above 600° C. while running a rich air-fuel mixture, typically by post fuel injection. A difficulty here is however that the temperature of the NOx trap should not rise up to levels where it may damage the latter. Therefore, as for the regeneration mode, in desulfation mode the engine is operated so that the exhaust air fuel mixture is alternately lean and rich in order to limit the temperature.
U.S. Pat. No. 7,036,489 relates to a NOx trap desulfation process employing an on-board reformer producing hydrogen and carbon monoxide to control the operating air-fuel ratio at the NOx trap.